The present invention relates to a method of fabricating a nitride semiconductor device such as a semiconductor laser diode expected to be applied to the fields of optical information processing and the like.
Recently, a nitride semiconductor of a group III-V compound, that is, a compound including nitride (N) as a group V element, is regarded as a promising material for a short-wavelength light emitting device due to its large energy gap. In particular, a gallium nitride-based compound semiconductor (AlxGayInzN, wherein 0≦x, y, z≦1 and x+y+z=1) has been earnestly studied and developed, resulting in realizing a practical blue or green light emitting diode (LED) device. Furthermore, in accordance with capacity increase of an optical disk unit, a semiconductor laser diode lasing at a wavelength of approximately 400 nm is earnestly desired, and a semiconductor laser diode using a gallium nitride-based semiconductor is to be practically used.
Now, a conventional gallium nitride-based semiconductor laser diode will be described with reference to a drawing.
FIG. 11 shows the sectional structure of the conventional gallium nitride-based semiconductor laser diode showing laser action. As is shown in FIG. 11, the conventional semiconductor laser diode includes a buffer layer 302 of gallium nitride (GaN), an n-type contact layer 303 of n-type GaN, a first cladding layer 304 of n-type aluminum gallium nitride (AlGaN), a first light guiding layer 305 of n-type GaN, a multiple quantum well (MQW) active layer 306 including gallium indium nitride layers having different composition ratios of indium (Ga1-xInxN/Ga1-yInyN wherein 0<y<x<1), a second light guiding layer 307 of P-type GaN, a second cladding layer 308 of p-type AlGaN and a p-type contact layer 309 of p-type GaN successively formed on a substrate 301 of sapphire by, for example, metal organic vapor phase epitaxial growth (MOVPE).
An upper portion of the second cladding layer 308 and the p-type contact layer 309 are formed into a ridge with a width of approximately 3 through 10 μm. A lamination body including the MQW active layer 306 formed on the semiconductor substrate 301 is etched so as to expose part of the n-type contact layer 303, and the top face and the side faces of the etched lamination body are covered with an insulating film 310. In a portion of the insulating film 310 above the p-type contact layer 309, a stripe-shaped opening is formed, and a p-side electrode 311 in ohmic contact with the p-type contact layer 309 through the opening is formed over a portion of the insulating film 310 above the ridge. Also, on a portion of the n-type contact layer 303 not covered with the insulating film 310, an n-side electrode 312 in ohmic contact with the n-type contact layer 303 is formed.
In the semiconductor laser diode having the aforementioned structure, when a predetermined voltage is applied to the p-side electrode 311 with the n-side electrode 312 grounded, optical gain is generated within the MQW active layer 306, so as to show laser action at a wavelength of approximately 400 nm. The wavelength of the laser action depends upon the composition ratios x and y or the thicknesses of the Ga1-xInxN and Ga1-yInyN layers included in the MQW active layer 306. At present, the semiconductor laser diode having this structure has been developed to show continuous laser action at room temperature or more.
It is generally well known that the growth temperature for growing a nitride semiconductor crystal by the MOVPE is changed in accordance with the composition ratio of a group III element introduced into gallium nitride (GaN).
It is reported that, in growing a semiconductor of, for example, gallium indium nitride (GaInN), nitrogen (N2) is preferably used as a material carrier gas with the growth temperature for the semiconductor set to approximately 800° C. (Applied Physics Letters, Vol. 59, pp. 2251-2253, 1991).
On the other hand, it is also known that the first and second cladding layers 304 and 308 and the first and second light guiding layer 305 and 307 not including indium are preferably grown at a growth temperature of 1000° C. or more with hydrogen (H2) used as a carrier gas.
The fabrication processes for these semiconductor layers are disclosed in, for example, Japanese Laid-Open Patent Publication No. 6-196757 or 6-177423.
The outline of the processes will now be described with reference to FIG. 11.
First, with hydrogen introduced onto a substrate 301, the principal plane of the substrate 301 is subjected to a heat treatment at a temperature of approximately 1050° C. Then, after lowering the substrate temperature to approximately 510° C., ammonia (NH3) and trimethylgallium (TMG), that is, mutually reactive gases, are introduced onto the substrate 301, so as to grow a buffer layer 302. Thereafter, with the introduction of TMG stopped, the substrate temperature is increased to approximately 1030° C., and TMG and monosilane (SiH4) are introduced onto the substrate 301 with hydrogen used as a carrier gas, thereby successively growing an n-type contact layer 303, a first cladding layer 304 and a first light guiding layer 305, whereas trimethylaluminum (TMA) is additionally introduced as a group III material gas in growing the first cladding layer 304.
Next, the introduction of the material gases is stopped, the substrate temperature is lowered to approximately 800° C., and the carrier gas is changed to nitrogen. Subsequently, trimethylindium (TMI) and TMG are introduced onto the substrate 301 as the group III material gases, thereby growing a MQW active layer 306.
Then, the introduction of the group III material gases is stopped, the substrate temperature is increased to approximately 1020° C. and a group III material gas, that is, TMG and TMA if necessary, and cyclopentadienylmagnesium (Cp2Mg) including a p-type dopant are introduced onto the substrate 301, thereby successively growing a second light guiding layer 307, a second cladding layer 308 and a p-type contact layer 309.
After growing the MQW active layer 306, as a protection film for the active layer in increasing the temperature from 800° C. to 1020° C., a semiconductor layer of GaN is formed according to the description of Japanese Laid-Open Patent Publication No. 9-186363 or a semiconductor layer of Al0.2Ga0.8N is formed according to description of, for example, Japanese Journal of Applied Physics (Vol. 35, pp. L74-L76, 1996).
In general, the vapor phase epitaxial growth is conducted in an atmosphere of reduced pressure lower than the atmospheric pressure, the atmospheric pressure or increased pressure lower than approximately 1.5 atm.
A technique to suppress defects from occurring on an interface between a substrate and gallium nitride by growing gallium nitride on a substrate of sapphire by selective growth or the like is recently tried. It is reported with respect to this technique that gallium nitride with a flat face and high crystal quality can be obtained by conducting the vapor phase epitaxial growth under reduced pressure in particular.
As described so far, as a characteristic of growth of a gallium nitride-based semiconductor, different carrier gases are used in growing a layer including indium, namely, the MQW active layer 306, and layers not including indium, such as the first cladding layer 304 and the first light guiding layer 305. In general, nitrogen is used for growing the former layer and hydrogen is used for growing the latter layers.
Accordingly, in the fabrication of a semiconductor laser diode, particularly in forming a multilayer structure including double heterojunction layers sandwiching an active layer by the vapor phase epitaxial growth, it is necessary to change the carrier gas before and after forming the active layer. Also, the substrate temperature is changed at the same time. In changing the carrier gas, the introduction of the group III material gases such as TMG is stopped, and the substrate is placed in an equilibrium state where no crystal grows.
However, in the aforementioned conventional method of fabricating a nitride semiconductor device, the crystal face of the grown semiconductor layer is exposed to a high temperature of approximately 1000° C. and reduced pressure lower than 1 atm while the substrate is placed in the equilibrium state where the introduction of the group III material gases is stopped. As a result, there arises a problem that constituent elements are released (re-evaporated) from the crystal face.
In particular, quality degradation of the first cladding layer 304 and the first light guiding layer 305 formed below the MQW active layer 306, particularly the first cladding layer 305 including 10% of aluminum in the aforementioned publication, leads to quality degradation of the MQW active layer 306. This degradation results in lowering the luminous efficiency and degrading operation characteristics, for example, increasing a threshold current, of the resultant light emitting diode or semiconductor laser diode.
Furthermore, it is recently reported in Journal of Electronic Materials (Vol. 28, No. 3, pp. 287-289, 1999) that when gallium nitride is grown under increased pressure, an etch pit density can be reduced so as to suppress point defects.
On the other hand, the present inventors have found the following problem: When a nitride semiconductor is simply grown under increased pressure exceeding the atmospheric pressure in the above-described equilibrium state, the concentration of material gases is so increased that vapor phase reactions of ammonia with trimethylaluminum and cyclopentadienylmagnesium are caused, resulting in producing intermediate reaction products through these intermediate reactions.
Accordingly, the material gases cannot be efficiently supplied onto the growth face of a crystal on the substrate, resulting in extremely lowering the growth rate or preventing magnesium (Mg), that is, the p-type dopant, from being introduced into the crystal.
Furthermore, when the flow rate of a carrier gas for carrying the material gases is increased for avoiding the production of the intermediate reaction product, the amount of gases flowing through a reaction tube is so large that vortexes and convections are caused in the air flow within the reaction tube. As a result, the crystal cannot be grown under stable conditions.